Multitone Interference Detection after Dehopping Process in Slow FH/BFSK Systems over Rayleigh Fading Channels Aye Aung, Kah Chan Teh, and Kwok Hung Li School of Electrical and Electronic Engineering Nanyang Technological University, Singapore 639798 E-mails: {ayeaung, ekcteh, ekhli }@ntu.edu.sg Abstract—This paper presents an algorithm to detect unknown multitone interference (MTI) which is commonly encountered in slow frequency-hopped spread spectrum (FH-SS) systems over a Rayleigh fading channel. The least-squares (LS) autoregressive (AR) spectral estimation method is used in our algorithm. The proposed algorithm performs the detection process after dehopping, hence, it can be used to provide the side information whether the MTI is present or absent for optimum maximum- likelihood (ML) FH-SS receivers. The fast Fourier transform (FFT) can also be used to implement this detection algorithm. Computer simulations are conducted to evaluate detection per- formance of both algorithms and comparisons are presented. Numerical results show that the MTI can be detected by both algorithms even at very low interference power (10 dB lower than signal power). Performance comparisons reveal that the proposed algorithm using the AR method is able to locate the MTI within the desired FH band more precisely than the FFT- based algorithm. I. I NTRODUCTION Frequency-hopped spread spectrum (FH-SS) system has been an attractive spread spectrum (SS) communication pro- tocol which is widely used in military and commercial appli- cations due to its capability to provide reliable information transmission over the channels in the presence of strong noise and interference. Regardless of this robustness, per- formance of FH-SS systems can be degraded by the inten- tional/unintentional interference. Multitone interference (MTI) and partial-band noise interference (PBNI) are two principal categories of interference waveforms encountered in FH-SS systems [1], [2]. Under the same interference power, the MTI approach is more effective than the PBNI strategy due to its efficient power utilization, which can actually cause critical harm to the FH-SS systems. The most commonly used data modulation for FH-SS systems is M-ary frequency-shift-keying (MFSK) [1], [3]. For FH/MFSK systems, the carrier frequency is randomly hopped over a total SS bandwidth,   , according to a pseudorandom (PN) sequence. Based on the hopping rate with respect to the information symbol rate, there are two types of FH-SS systems, namely fast FH-SS (FFH-SS) system and slow FH- SS (SFH-SS) system. In the former, the signal is hopping much faster than the information symbol rate whereas in the latter, the hopping rate is slower than the symbol rate. In this paper, we consider a slow FH-SS system which adopts a binary FSK (BFSK) modulation scheme. In SFH-SS systems, more than one symbol are transmitted in one frequency hopping interval, hence, the system will incur more than one symbol error if strong MTI is present in a desired FH band. Therefore, information of MTI is essential for the purpose of mitigating the interference in order to improve the bit-error-rate (BER) performance of FH receivers. Besides, it is known that the receiver with perfect side information can achieve a better performance than those without side information [2]–[4]. Therefore, it is crucial to detect the unknown MTI encountered in SFH-SS systems in order to mitigate it as much as possible before demodulating the desired data. In literature, very few papers have been devoted to discuss on these detection issues in FH-SS systems [5], [6]. In [5], the wavelet packet transform was used for interference detection in FH-SS systems and the FFT was employed in [6] to estimate the interference. On the other hand, various types of spectral estimation methods [7]–[10] have been proposed to detect sinusoidal signals in literature. These methods can be used to detect the MTI in SFH-SS systems. But if the MTI is present in the same location as the signaling tones, it will be difficult to distinguish the MTI from the signal of interest. In this paper, we propose a detection algorithm to detect and identify tone interference encountered in slow FH-SS systems over a Rayleigh fading channel by using an autoregressive (AR) spectral estimation method. The parametric AR method is chosen due to the fact that the parametric AR spectral estimation approach is able to provide a good accuracy and higher frequency resolution than non-parametric approach which is based on the discrete Fourier transform (DFT) [7], [11]. Moreover, the DFT frequency resolution is limited by the length of data observation window which is used to acquire the spectrum whereas the resolution for the parametric method is merely dependent on the value of signal-to-noise ratio (SNR) instead of the observation interval. Our proposed detection algorithm is performed in frequency domain. The received signal is first downconverted and band-pass (  ) filtered. After dehopping, the signal is passed through an analog band- pass filter (BPF) with narrowband bandwidth   and then the detection process is carried out by using the proposed algorithm. As such we can limit the amount of noise to the analog-to-digital converter (ADC) input since sampling 978-1-4577-0031-6/11/$26.00 ©2011 IEEE ICICS 2011